In hot melt adhesive guns, such as shown in U.S. Pat. No. 3,877,610, the hot melt adhesive is in the form of a cylindrical cartridge which is introduced into the gun chamber for melting and dispensing out of the gun nozzle. The adhesive cartridge is formed of a homogenous adhesive material having an intended melting temperature and adhesive properties. Many adhesive materials are relatively soft at room temperature and have tacky surface characteristics and are difficult to form to a predetermined outside dimension. The outside dimension of an adhesive cartridge, usually the diameter of a cylindrical cartridge, must be maintained within close tolerances to not impair proper gun operation. If the cartridge diameter is too large, the cartridge will not be freely movable in the gun chamber and can prevent propulsion of the cartridge into engagement with the heater within the chamber for melting of the forward portion of the cartridge. If the cartridge diameter is too small, melted adhesive in the gun chamber can tend to back-up into the clearance space between the undersized cartridge and the surrounding chamber wall. As a result, the chamber can become clogged, and the cartridge can jam within the chamber, preventing further cartridge propulsion for heating and dispensing.
An object of the invention is to provide a hot melt adhesive cartridge having a thermoplastic outer layer which provides an accurately dimensioned cartridge having a non-stick surface characteristic. Such a cartridge having an outer, non-stick, thermoplastic layer is known but has been made by a batch technique which does not lend itself to continuous and inexpensive mass production.
According to the technique previously employed by the applicant herein, a non-stick, thermoplastic sleeve is disposed within a rigid paperboard or other container, the sleeve then being filled with molten hot melt adhesive. After solidification of the adhesive, the adhesive and surrounding sleeve is removed from the supporting container. This technique is limited to the formation of relatively short lengths of adhesive of usable cartridge size, or of a larger size which is then cut into usable cartridges. In addition, this technique requires the use of a support container which must later be removed.
It would be useful to form a hot melt cartridge with a non-stick outer layer by a continuous extrusion process; however, known extrusion processes have not been satisfactory by reason of the particular characteristics of hot melt adhesives. In one known extrusion process commonly referred to as post-coating, a thermoplastic material is extruded through a shape die to form an intended configuration. The extrusion then passes through a second die for application of a coating thereon.
This known technique is not suitable for hot melt materials due to the tackiness of the material, its low viscosity and very slow setting characteristics. Shape integrity cannot be maintained especially for larger diameters, and therefore an accurately sized adhesive cartridge cannot be formed. In another known extrusion process referred to as co-extrusion, two extruders feed a cross-head die to simultaneously extrude a core and surrounding outer shell. In this latter process, the core material must be extruded at a much lower temperature than the shell material to maintain continuous shape integrity and avoid deformation of the shell. The elevated temperatures necessary to form a homogeneous hot melt core render this known extrusion process unsuitable.